1. Field of the Invention
The invention in general relates to the field of medical pneumatic tourniquets, and more particularly to a tourniquet which is automatically controlled to maintain the desired tourniquet pressure.
2. Description of the Prior Art
Tourniquets are used in medicine to stop the flow of blood into a patient's limb. A typical use is to provide a "bloodless field" in which surgical procedures may be performed on a limb, without the field being obscured by blood, and without loss of blood to the patient. Tourniquets are also used in emergency procedures to prevent loss of blood when a limb is damaged or in any other instance where it is desired to prevent blood flow to a limb.
Tourniquets conventionally consist of an inflatable cuff which is wrapped around the patient's limb, and a source of compressed gas for inflating the cuff. In order to successfully stop the flow of blood, the pressure in the cuff must be above the highest level of the patient's blood pressure. However, it should not be so high that it damages the tissue of the limb. Thus in conventional tourniquets a pressure gauge for measuring the cuff pressure, and a pressure regulating mechanism is provided.
Up until very recently, conventional pneumatic tourniquets have been manually controlled mechanical devices. That is, a mechanical pressure gauge indicated the pressure visually and the pressure was adjusted by an attendant using a mechanical valve.
Recently, an automatic tourniquet for medical use has been created by Dr. James A. McEwen. This tourniquet is described in U.S. patent application Ser. No. 193,145 filed Oct. 2, 1980. An improved automatic tourniquet is described in U.S. patent application Ser. No. 337,152 filed Jan. 5, 1982, also by Dr. James McEwen. In these automatic tourniquets, the pressure in the cuff is continuously sensed and adjusted by a microprocessor. In addition, the microprocessor activates alarms if a dangerous pressure condition is sensed. It has been found that such automatic tourniquets systems not only are much more convenient in the operating room setting, but also significantly reduce the incidence of limb paralysis, nerve damage and other injuries attributable to tourniquets.
The above-described prior art automatic tourniquets regulate the tourniquet cuff pressure by actuating pressure correcting means if the pressure rises above or falls below a fixed limit. The tourniquets compare the sensed pressure to the desired pressure and if the sensed pressure exceeds a fixed limit, for example 6 mmHg above the selected pressure, then the cuff is deflated a fixed amount, while if the sensed pressure is below the desired pressure by a prescribed limit, the cuff is inflated for a fixed time period, such as two seconds.
The above-described manner of compensating for pressure errors in the prior art system can lead to a cuff pressure which oscillates about the desired pressure.